Senescent Cell Biomarkers

ABSTRACT

The invention relates to senescent cell biomarkers and the uses thereof. The invention also extends to methods and kits for detecting senescence, and drug conjugates and pharmaceutical compositions for killing senescent cells.

The present invention relates to senescent cells, and in particular, to biomarkers that are assembled into, or associated with, the plasma membrane of senescent cells, and to methods of identifying senescent cells. The invention also extends to therapies, compositions and methods for targeting and killing senescent cells, and uses thereof.

Apoptosis and senescence have been proposed to be the two main processes that prevent the emergence of transformed cells. Senescence is defined as a permanent cell cycle arrest in which cells remain metabolically active and adopt characteristic phenotypic changes. Senescent cells often appear multinucleated, large and extended, and exhibit spindle and vacuolisation features. The establishment of this phenotype is believed to be either the result of telomere shortening after a number of cell divisions (replicative senescence) or a response to stress stimuli (stress-induced premature senescence, SIPS). Expression of oncogenes, such as Ras, cyclin E, E2F3 and Raf can also trigger senescence, which supports the tumour suppressing properties of senescence.

The presence of senescent cells in vivo is often observed in the pre-malignant stages of a tumour and they progressively disappear, suggesting that the senescent barrier needs to be overcome in order to progress into full malignancy. Moreover, cell senescence has long been associated with age-dependent organismal changes, since accumulation of senescent cells has been shown to contribute to the functional impairment of different organs. This has led to the hypothesis that senescence is an antagonistically pleiotropic process, with beneficial effects in the early decades of life of the organism as a tumour suppressor but that can be detrimental to fitness and survival in later stages due to its involvement in ageing.

One of the well-known features of both replicative and stress-induced senescence is the participation of the p53-p21 and/or p16-RB pathways. In vivo suppression of p53, and/or its upstream regulator ARF, is enough to prevent senescence in some models.

However, other cell types rely primarily on p16 for senescence induction. The p53 target gene, p21, has often been considered critical for establishing senescence, whereas p16 could be more involved in the maintenance of the phenotype, together with an increase in intracellular Reactive Oxygen Species (ROS). Other genes upregulated in senescent cells are PPP1A, Smurf2 and PGM.

Cellular senescence is also associated with the secretion of growth factors, chemokines and cytokines, known as the senescence-associated secretary phenotype (SASP). SASPs have shown an effect on cell proliferation and angiogenesis, as well as in promoting ageing. Also, SASP can induce migration of leukocytes and tumour cells, which in turn may induce tumour metastasis. Increased expression of intracellular and/or secreted proteins has often been used as a surrogate marker of senescence, although it is not a specific measurement.

Senescent cells also display different modifications in the organisation of chromatin that can help identify them. In normal cells, DNA staining reveals completely uniform colour outlines, whereas senescent cells usually show dot-like patterns, known as senescence-associated heterochromatic foci (SAHF). This phenomenon is due to intensive remodelling in the chromatin, which results in less susceptibility for digestion by nucleases. SAHF development is not necessary for senescence to occur, and this depends primarily on cell types and senescent stimuli.

Apart from all this, the most distinctive measurable feature of senescent cells is the presence of β-galactosidase enzymatic activity. This enzyme normally displays activity at pH 4.0 within lysosomes, but in senescent cells it is also active at pH 6.0. This phenomenon is termed senescence associated-β-galactosidase (SA-β-Gal). Although the reasons for this are not completely clear, it is thought to be due to an enlargement in the structure of lysosomes in senescent cells. SA-β-Gal has not been shown to have any role in the establishment or maintenance of the senescent phenotype. Although it is currently the standard for the detection of senescent cells, high cell confluence and treatment with hydrogen peroxide can also stimulate SA-β-Gal activity, leading to many false positives. None of the currently available markers satisfactorily or conclusively identify senescent cells in vivo or in vitro, which underscores the need for better characterization tools.

Despite the considerable knowledge accumulated in the fifty years since Leonard Hayflick first described the phenomenon of senescence, the molecular pathways involved in the establishment and maintenance of the senescent phenotype still have not been fully characterized. For instance, little is known about the profile of proteins specifically expressed in the membrane of senescent cells, which could be critical for the immune clearance of senescent cells observed in certain situations.

There is therefore a need for more specific and sensitive senescent cell biomarkers.

The inventors have studied the expression profile of plasma membrane proteins in senescent cells in order to identify novel markers that could be easily recognized and propose potential effectors and modulators of the senescent pathway. Ten novel specific markers of senescence were validated, and two of these were selected in order to develop a fast and straightforward FACS-based approach to identify senescent cells. The results described herein will facilitate the study of senescent cells and provide new insights on pathways that contribute to this mechanism. In addition, identification of these ten new senescence cell biomarkers will be very useful in targeting senescent cells for treating conditions associated with cell senescence.

Thus, according to a first aspect of the invention, there is provided the use of at least one polypeptide selected from DEP-1, NTAL, EBP50, STX4, VAMP3, ARMCX-3, LANCL1, B2MG, PLD3 and VPS26A, or a variant or fragment thereof, as a senescent cell biomarker.

Advantageously, all of the senescent cell biomarkers used in accordance with the first aspect of the invention are associated with the plasma membrane of senescent cells. The biomarkers exhibit low or non-existent basal expression in non-senescent cells and/or are inducible in senescent cells. Consequently, these biomarkers are extremely specific and sensitive to detection. Some of the biomarkers according to the invention contain at least one domain or epitope, which is exposed on the extracellular surface of senescent cells (see Table 1), whereas the remaining biomarkers are expressed intracellularly and are associated with the plasma membrane of senescent cells. Biomarkers that are expressed on the surface of senescent cells enable senescent cells to be detected more quickly and easily compared to the known, and more widely used senescent cell biomarker, SA-β-Gal, which is considered an unreliable biomarker. Therefore, it will be appreciated that the senescent biomarkers disclosed herein can be readily detected using a variety of simple, conventional techniques known in the art.

TABLE 1 Subcellular location and membrane topology of biomarkers showing increased expression in senescent cells Senescent biomarker Subcellular location DEP-1 Integral transmembrane protein of plasma membrane NTAL Integral transmembrane protein of plasma membrane EBP50 Peripheral membrane protein associated with the cytoplasmic face of the plasma membrane (intracellular) STX4 Membrane anchored protein with a large intracellular domain and no extracellular domain VAMP3 Membrane anchored protein with a large intracellular domain and no extracellular domain ARMCX-3 Integral plasma membrane protein with two transmembrane helices and two extracellular domains LANCL1 Integral plasma membrane protein with six transmembrane helices and three extracellular domains B2MG Secreted protein (extracellular) found associated with the extra:: cytoplasmic face of the plasma membrane PLD3 Trans membrane protein of plasma membrane with a single extracellular domain VPS26A Peripheral membrane protein associated with the cytoplasmic face of the plasma membrane (intracellular)

DEP-1 is an integral transmembrane membrane protein of the plasma membrane. The amino acid sequence of DEP-1 (Accession code: Q12913; also known as CD148 or PTPRJ) is referred to herein as SEQ ID No.1, as follows:

[SEQ ID No. 1] MKPAAREARL PPRSPGLRWA LPLLLLLLRL GQILCAGGTP SPIPDPSVAT VATGENGITQ ISSTAESFHK QNGTGTPQVE TNTSEDGESS GANDSLRTPE QGSNGTDGAS QKTPSSTGPS PVFDIKAVSI SPTNVILTWK SNDTAASEYK YVVKHKMENE KTITVVHQPW CNITGLRPAT SYVFSITPGI GNETWGDPRV IKVITEPIPV SDLRVALTGV RKAALSWSNG NGTASCRVLL ESIGSHEELT QDSRLQVNIS GLKPGVQYNI NPYLLQSNKT KGDPLGTEGG LDASNTERSR AGSPTAPVHD ESLVGPVDPS SGQQSRDTEV LLVGLEPGTR YNATVYSQAA NGTEGQPQAI EFRTNAIQVF DVTAVNISAT SLTLIWKVSD NESSSNYTYK IHVAGETDSS NLNVSEPRAV IPGLRSSTFY NITVCPVLGD IEGTPGFLQV HTPPVPVSDF RVTVVSTTEI GLAWSSHDAE SFQMHITQEG AGNSRVEITT NQSIIIGGLF PGTKYCFEIV PKGPNGTEGA SRTVCNRTVP SAVFDIHVVY VTTTEMWLDW KSPDGASEYV YHLVIESKHG SNHTSTYDKA ITLQGLIPGT LYNITISPEV DHVWGDPNST AQYTRPSNVS NIDVSTNTTA ATLSWQNFDD ASPTYSYCLL IEKAGNSSNA TQVVTDIGIT DATVTELIPG SSYTVEIFAQ VGDGIKSLEP GRKSFCTDPA SMASFDCEVV PKEPALVLKW TCPPGANAGF ELEVSSGAWN NATHLESCSS ENGTEYRTEV TYLNFSTSYN ISITTVSCGK MAAPTRNTCT TGITDPPPPD GSPNITSVSH NSVKVKFSGF EASHGPIKAY AVILTTGEAG HPSADVLKYT YEDFKKGASD TYVTYLIRTE EKGRSQSLSE VLKYEIDVGN ESTTLGYYNG KLEPLGSYRA CVAGFTNITF HPQNKGLIDG AESYVSFSRY SDAVSLPQDP GVICGAVFGC IFGALVIVTV GGFIFWRKKR KDAKNNEVSF SQIKPKKSKL IRVENFEAYF KKQQADSNCG FAEEYEDLKL VGISQPKYAA ELAENRGKNR YNNVLPYDIS RVKLSVQTHS TDDYINANYM PGYHSKKDFI ATQGPLPNTL KDFWRMVWEK NVYAIIMLTK CVEQGRTKCE EYWPSKQAQD YGDITVAMTS EIVLPEWTIR DFTVKNIQTS ESHPLRQFHF TSWPDHGVPD TTDLLINFRY LVRDYMKQSP PESPILVHCS AGVGRTGTFI AIDRLIYQIE NENTVDVYGI VYDLRMHRPL MVQTEDQYVF LNQCVLDIVR SQKDSKVDLI YQNTTAMTIY ENLAPVTTFG KTNGYIA

Preferably, the extracellular domain of DEP-1 is used as a biomarker of senescent cells. The amino acid sequence of the extracellular domain of DEP-1 is referred to herein as SEQ ID No. 2, as follows:

[SEQ ID No. 2] AGGTPSPIPD PSVATVATGE NGITQISSTA ESFHKQNGTG TPQVETNTSE DGESSGANDS LRTPEQGSNG TDGASQKTPS STGPSPVFDI KAVSISPTNV ILTWKSNDTA ASEYKYVVKH KMENEKTITV VHQPWCNITG LRPATSYVFS ITPGIGNETW GDPRVIKVIT EPIPVSDLRV ALTGVRKAAL SWSNGNGTAS CRVLLESIGS HEELTQDSRL QVNISGLKPG VQYNINPYLL QSNKTKGDPL GTEGGLDASN TERSRAGSPT APVHDESLVG PVDPSSGQQS RDTEVLLVGL EPGTRYNATV YSQAANGTEG QPQAIEFRTN AIQVFDVTAV NISATSLTLI WKVSDNESSS NYTYKIHVAG ETDSSNLNVS EPRAVIPGLR SSTFYNITVC PVLGDIEGTP GFLQVHTPPV PVSDFRVTVV STTEIGLAWS SHDAESFQMH ITQEGAGNSR VEITTNQSII IGGLFPGTKY CFEIVPKGPN GTEGASRTVC NRTVPSAVFD IHVVYVTTTE MWLDWKSPDG ASEYVYHLVI ESKHGSNHTS TYDKAITLQG LIPGTLYNIT ISPEVDHVWG DPNSTAQYTR PSNVSNIDVS TNTTAATLSW QNFDDASPTY SYCLLIEKAG NSSNATQVVT DIGITDATVT ELIPGSSYTV EIFAQVGDGI KSLEPGRKSF CTDPASMASF DCEVVPKEPA LVLKWTCPPG ANAGFELEVS SGAWNNATHL ESCSSENGTE YRTEVTYLNF STSYNISITT VSCGKMAAPT RNTCTTGITD PPPPDGSPNI TSVSHNSVKV KFSGFEASHG PIKAYAVILT TGEAGHPSAD VLKYTYEDFK KGASDTYVTY LIRTEEKGRS QSLSEVLKYE IDVGNESTTL GYYNGKLEPL GSYRACVAGF TNITFHPQNK GLIDGAESYV SFSRYSDAVS LPQDPGVICG

NTAL is an integral transmembrane membrane protein of the plasma membrane. The amino acid sequence of NTAL (Accession code: Q9GZY6; also known as LAT2) is referred to herein as SEQ ID No.3, as follows:

[SEQ ID No. 3] MSSGTELLWP GAALLVLLGV AASLCVRCSR PGAKRSEKIY QQRSLREDQQ SFTGSRTYSL VGQAWPGPLA DMAPTRKDKL LQFYPSLEDP ASSRYQNFSK GSRHGSEEAY IDPIAMEYYN WGRFSKPPED DDANSYENVL ICKQKTTETG AQQEGIGGLC RGDLSLSLAL KTGPTSGLCP SASPEEDEES EDYQNSASIH QWRESRKVMG QLQREASPGP VGSPDEEDGE PDYVNGEVAA TEA

Preferably, the extracellular domain of NTAL is used as a biomarker of senescent cells. The amino acid sequence of the extracellular domain of NTAL is referred to herein as SEQ ID No.4, as follows:

[SEQ ID No. 4] MSSGTE

EBP50 is an intracellular protein associated with the cytoplasmic face of the plasma membrane. The amino acid sequence of EBP50 (Accession code: 014745; also known as NHERF1) is referred to herein as SEQ ID No.5, as follows:

[SEQ ID No. 5] MSADAAAGAP LPRLCCLEKG PNGYGFHLHG EKGKLGQYIR LVEPGSPAEK AGLLAGDRLV EVNGENVEKE THQQVVSRIR AALNAVRLLV VDPETDEQLQ KLGVQVREEL LRAQEAPGQA EPPAAAEVQG AGNENEPREA DKSHPEQREL RPRLCTMKKG PSGYGFNLHS DKSKPGQFIR SVDPDSPAEA SGLRAQDRIV EVNGVCMEGK QHGDVVSAIR AGGDETKLLV VDRETDEFFK KCRVIPSQEH LNGPLPVPFT NGEIQKENSR EALAEAALES PRPALVRSAS SDTSEELNSQ DSPPKQDSTA PSSTSSSDPI LDFNISLAMA KERAHQKRSS KRAPQMDWSK KNELFSNL

STX4 is a plasma membrane anchored protein with a large intracellular domain and no extracellular domain. The amino acid sequence of STX4 (Accession code: Q12846) is referred to herein as SEQ ID No.6, as follows:

[SEQ ID No. 6] MRDRTHELRQ GDDSSDEEDK ERVALVVHPG TARLGSPDEE FFHKVRTIRQ TIVKLGNKVQ ELEKQQVTIL ATPLPEESMK QELQNLRDEI KQLGREIRLQ LKAIEPQKEE ADENYNSVNT RMRKTQHGVL SQQFVELINK CNSMQSEYRE KNVERIRRQL KITNAGMVSD EELEQMLDSG QSEVFVSNIL KDTQVTRQAL NEISARHSEI QQLERSIREL HDIFTFLATE VEMQGEMINR IEKNILSSAD YVERGQEHVK TALENQKKAR KKKVLIAICV SITVVLLAVI IGVTVVG

VAMP3 is a plasma membrane anchored protein with a large intracellular domain and no extracellular domain. The amino acid sequence of VAMP3 (Accession code: Q15836; also known as Cellubrevin) is referred to herein as SEQ ID No.7, as follows:

[SEQ ID No. 7] MSTGPTAATG SNRRLQQTQN QVDEVVDIMR VNVDKVLERD QKLSELDDRA DALQAGASQF ETSAAKLKRK YWWKNCKMWA IGITVLVIFI IIIIVWVVSS

ARMCX-3 is an integral plasma membrane protein with two transmembrane helices and two extracellular domains. The amino acid sequence of ARMCX-3 (Accession code: Q9UH62; also known as ALEX3) is referred to herein as SEQ ID No.8, as follows:

[SEQ ID No. 8] MGYARKVGWV TAGLVIGAGA CYCIYRLTRG RKQNKEKMAE GGSGDVDDAG DCSGARYNDW SDDDDDSNES KSIVWYPPWA RIGTEAGTRA RARARARATR ARRAVQKRAS PNSDDTVLSP QELQKVLCLV EMSEKPYILE AALIALGNNA AYAFNRDIIR DLGGLPIVAK ILNTRDPIVK EKALIVLNNL SVNAENQRRL KVYMNQVCDD TITSRLNSSV QLAGLRLLTN MTVTNEYQHM LANSISDFFR LFSAGNEETK LQVLKLLLNL AENPAMTREL LRAQVPSSLG SLFNKKENKE VILKLLVIFE NINDNFKWEE NEPTQNQFGE GSLFFFLKEF QVCADKVLGI ESHHDFLVKV KVGKFMAKLA EHMFPKSQE

Preferably, both extracellular domains of ARMCX-3 are used as a biomarker of senescent cells. The amino acid sequence of the first extracellular domain of ARMCX-3 is referred to herein as SEQ ID No. 9, as follows:

[SEQ ID No. 9] MGYARK

The amino acid sequence of the second extracellular domain of ARMCX-3 is referred to herein as SEQ ID No. 10, as follows:

[SEQ ID No. 10] NRDIIRDLGGLPIVAKILNTRDPIVKEKALIVLNNLSVNAENQRRLKVYM NQVCDDTITSRLNSSVQLAGLRLLTNMTVTNEYQHMLANSISDFFRLFSA GNEETKLQVLKLLLNLAENPAMTRELLRAQVPSSLGSLFNKKENKEVILK LLVIFENINDNFKWEENEPTQNQFGEGSLFFFLKEFQVCADKVLGIESHH DFLVKVKVGKFMAKLAEHMFPKSQE

LANCL1 is an integral plasma membrane protein with six transmembrane helices and three extracellular domains. The amino acid sequence of LANCL1 (Accession code: O43813) is referred to herein as SEQ ID No.11, as follows:

[SEQ ID No. 11] MAQRAFPNPY ADYNKSLAEG YFDAAGRLTP EFSQRLTNKI RELLQQMERG LKSADPRDGT GYTGWAGIAV LYLHLYDVFG DPAYLQLAHG YVKQSLNCLT KRSITFLCGD AGPLAVAAVL YHKMNNEKQA EDCITRLIHL NKIDPHAPNE MLYGRIGYIY ALLFVNKNFG VEKIPQSHIQ QICETILTSG ENLARKRNFT AKSPLMYEWY QEYYVGAAHG LAGIYYYLMQ PSLQVSQGKL HSLVKPSVDY VCQLKFPSGN YPPCIGDNRD LLVHWCHGAP GVIYMLIQAY KVFREEKYLC DAYQCADVIW QYGLLKKGYG LCHGSAGNAY AFLTLYNLTQ DMKYLYRACK FAEWCLEYGE HGCRTPDTPF SLFEGMAGTI YFLADLLVPT KARFPAFEL

Preferably, all three extracellular domains of LANCL1 may be used as a biomarker of senescent cells. The amino acid sequence of the first extracellular domain of LANCL1 is referred to herein as SEQ ID No.12, as follows:

[SEQ ID No. 12] DVFGDPAYLQLAHGYVKQSLNCLTKR

The amino acid sequence of the second extracellular domain of LANCL1 is referred to herein as SEQ ID No.13, as follows:

[SEQ ID No. 13] EKIPQSHIQQICETILTSGENLARKRNFTAKSPLMYEWYQEYYVGAAHGL AGIYYYLMQPSLQVSQGKLHSLVKPSVDYVCQLKFPSGNYPPCIGDNRD

The amino acid sequence of the third extracellular domain of LANCL1 is referred to herein as SEQ ID No.14, as follows:

[SEQ ID No. 14] DMKYLYRACKFAEWCLEYGEHGCRTPDTP

B2MG is a secreted protein associated with the extra-cytoplasmic surface of the plasma membrane. The amino acid sequence of B2MG (Accession code: P61769) is referred to herein as SEQ ID No.15, as follows:

[SEQ ID No. 15] MSRSVALAVL ALLSLSGLEA IQRTPKIQVY SRHPAENGKS NFLNCYVSGF HPSDIEVDLL KNGERIEKVE HSDLSFSKDW SFYLLYYTEF TPTEKDEYAC RVNHVTLSQP KIVKWDRDM

The first 20 amino acids of SEQ ID No.15 are the signal peptide of B2MG. The signal peptide of B2MG is responsible for directing B2MG to the plasma membrane of the cell for translocation across the plasma membrane to become a secreted protein. The amino acid sequence of B2MG without signal peptide is referred to herein as SEQ ID No.16, as follows:

[SEQ ID No. 16] IQRTPKIQVY SRHPAENGKS NFLNCYVSGF HPSDIEVDLL KNGERIEKVE HSDLSFSKDW SFYLLYYTEF TPTEKDEYAC RVNHVTLSQP KIVKWDRDM

PLD3 is a transmembrane protein with a single extracellular domain. The amino acid sequence of PLD3 (Accession code: Q8IV08) is referred to herein as SEQ ID No.17, as follows:

[SEQ ID No. 17] MKPKLMYQEL KVPAEEPANE LPMNEIEAWK AAEKKARWVL LVLILAVVGF GALMTQLFLW EYGDLHLFGP NQRPAPCYDP CEAVLVESIP EGLDFPNAST GNPSTSQAWL GLLAGAHSSL DIASFYWTLT NNDTHTQEPS AQQGEEVLRQ LQTLAPKGVN VRIAVSKPSG PQPQADLQAL LQSGAQVRMV DMQKLTHGVL HTKFWVVDQT HFYLGSANMD WRSLTQVKEL GVVMYNCSCL ARDLTKIFEA YWFLGQAGSS IPSTWPRFYD TRYNQETPME ICLNGTPALA YLASAPPPLC PSGRTPDLKA LLNVVDNARS FIYVAVMNYL PTLEFSHPHR FWPAIDDGLR RATYERGVKV RLLISCWGHS EPSMRAFLLS LAALRDNHTH SDIQVKLFVV PADEAQARIP YARVNHNKYM VTERATYIGT SNWSGNYFTE TAGTSLLVTQ NGRGGLRSQL EAIFLRDWDS PYSHDLDTSA DSVGNACRLL

Preferably, the extracellular domain of PLD3 is used as a biomarker of senescent cells. The amino acid sequence of the extracellular domain of PLD3 is referred to herein as SEQ ID No. 18, as follows:

[SEQ ID No. 18] QLFLWEYGDLHLFGPNQRPAPCYDPCEAVLVESIPEGLDFPNASTGNPS TSQAWLGLLAGAHSSLDIASFYWTLTNNDTHTQEPSAQQGEEVLRQLQT LAPKGVNVRIAVSKPSGPQPQADLQALLQSGAQVRMVDMQKLTHGVLsH TKFWVVDQTHEYLGSANMDWRSLTQVKELGVVMYNCSCLARDLTKIFEA YWFLGQAGSSIPSTWPRFYDTRYNQETPMEICLNGTPALAYLASAPPPL CPSGRTPDLKALLNVVDNARSFIYVAVMNYLPTLEFSHPHRFWPAIDDG LRRATYERGVKVRLLISCWGHSEPSMRAFLLSLAALRDNHTHSDIQVKL FVVPADEAQARIPYARVNHNKYMVTERATYIGTSNWSGNYFTETAGTSL LVTQNGRGGLRSQLEAIFLRDWDSPYSHDLDTSADSVGNACRLL

VPS26A is an intracellular protein associated with the cytoplasmic face of the plasma membrane. The amino acid sequence of VPS26A (Accession code: O75436) is referred to herein as SEQ ID No.19, as follows:

[SEQ ID No. 19] MSFLGGFFGP ICEIDIVLND GETRKMAEMK TEDGKVEKHY LFYDGESVSG KVNLAFKQPG KRLEHQGIRI EFVGQIELFN DKSNTHEFVN LVKELALPGE LTQSRSYDFE FMQVEKPYES YIGANVRLRY FLKVTIVRRL TDLVKEYDLI VHQLATYPDV NNSIKMEVGI EDCLHIEFEY NKSKYHLKDV IVGKIYFLLV RIKIQHMELQ LIKKEITGIG PSTTTETETI AKYEIMDGAP VKGESIPIRL FLAGYDPTPT MRDVNKKFSV RYFLNLVLVD EEDRRYFKQQ EIILWRKAPE KLRKQRTNFH QRFESPESQA SAEQPEM

Thus, in one embodiment, preferably at least one polypeptide sequence comprising an amino acid sequence substantially as set out in any one of SEQ ID Nos. 1 to 19, or a variant or fragment thereof, is used as a senescent cell biomarker.

The inventors have found that the proteins DEP-1, NTAL, EBP50, STX4, VAMP-3, PLD3 and ARMCX-3 were specifically expressed in all senescent cells, whereas B2MG, LANCL1 and VPS26A were up-regulated only in p16-induced senescence. Therefore, preferably one or more of DEP-1, NTAL, EBP50, STX4, VAMP-3, PLD3 and ARMCX-3, or a variant or fragment thereof, is used as a senescent cell biomarker. Hence, the at least one polypeptide sequence comprises an amino acid sequence substantially as set out in any one of SEQ ID Nos. 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 17 and 18, or a variant or fragment thereof.

It is preferred however that the extracellular domains of any of the proteins described herein is used as a senescent cell biomarker. The term “fragment thereof” can therefore refer to an extracellular domain of the protein. Accordingly, preferably at least one polypeptide sequence comprising an amino acid sequence substantially as set out in any one of SEQ ID Nos. 2, 4, 9, 10, 12, 13, 14, 16 and 18, or a variant or fragment thereof, is used as an extracellular biomarker of senescent cells. Accordingly, one or more of DEP-1, NTAL, B2MG, ARMCX-3, PLD3 and LANCL1, or a variant or fragment thereof, is used as an extracellular biomarker.

Most preferably, one or more of DEP-1, NTAL, ARMCX-3 and PLD3, or a variant or fragment thereof, is used as an extracellular biomarker. Hence, preferably the at least one polypeptide sequence comprises an amino acid sequence substantially as set out in any one of SEQ ID Nos. 2, 4, 9, 10 and 18, or a variant or fragment thereof.

As described in the Examples, the inventors have demonstrated that any of the ten senescent cell biomarkers described herein can be used to specifically detect senescent cells in a biological sample.

Therefore, according to a second aspect, there is provided a method of detecting a senescent cell in a sample, the method comprises detecting the expression, in the sample, of at least one senescent cell biomarker selected from DEP-1, NTAL, EBP50, STX4, VAMP3, ARMCX-3, LANCL1, B2MG, PLD3 and VPS26A, or a variant or fragment thereof, wherein an increased level of expression of the at least one biomarker or a variant or fragment thereof relative to the level of expression detected in a reference sample is an indication of a senescent cell present in the sample.

The more biomarkers (or variants or fragments thereof) that are detected in the sample, the greater the accuracy and reliability with which senescent cells can be identified. The method may therefore comprise detecting two or more biomarkers, or variants or fragments thereof, in the sample. In another embodiment, the method may comprise detecting three or more biomarkers, or variants or fragments thereof, in the sample. Preferably, the method comprises detecting four, five, six or more of the biomarkers, or variants or fragments thereof, described herein. Most preferably, the method comprises detecting the presence of DEP-1, NTAL, EBP50, STX4, VAMP-3, PLD3 and/or ARMCX-3, or a variant or fragment thereof. The method may comprise detecting one or more of the biomarkers, according to the invention, in a sample together with other known biomarkers, such as DCR-2, Notch-3 or ICAM-1.

The term “detecting” can refer, but is not limited, to the use of any one of the following conventional assays for detecting the presence of one or more of the biomarkers, or variants or fragments thereof, in a sample: flow cytometry; immunoassays, such as enzyme-linked immunosorbent assays (ELISAs), an enzyme immunoassay (EIAs), radioimmunoassay (RIAs), Western Blots, immuo-precipitation or immunohistochemistry; chromogenic (enzyme activity) assays; fluorometric imaging plate reader (FLIPR) assay; or high performance liquid chromatography (HPLC) tandem mass spectrometry (MS/MS).

Preferably, the senescence biomarker is detected using flow cytometry. Advantageously, flow cytometry can be used to measure and distinguish between cell surface and intracellular localisation of a biomarker protein in situ. Intracellular localisation of biomarkers, according to the invention, can be detected using flow cytometry by exposing the cells in the sample to a permeabilization agent, such as saponin, which permits entry of the cytometric antibodies into the target cells.

Alternatively, the presence of one or more of the senescence biomarkers may be detected in the sample by measuring their functional activity, e.g. by enzyme assay.

Alternatively, to measure the level of gene expression of the senescence biomarkers, cDNA may be generated from mRNA extracted from cells present in the sample, and primers designed to amplify test sequences using a quantitative form of Polymerase Chain Reaction.

The “sample” is preferably a bodily sample taken from a test subject. Detection for the presence of at least one senescent cell biomarker, or a variant or fragment thereof, in the sample, is therefore preferably carried out in vitro. The sample may comprise blood, plasma, serum, spinal fluid, urine, sweat, saliva, tears, breast aspirate, prostate fluid, seminal fluid, vaginal fluid, stool, cervical scraping, cytes, amniotic fluid, intraocular fluid, mucous, moisture in breath, animal tissue, cell lysates, tumour tissue, hair, skin, buccal scrapings, nails, bone marrow, cartilage, prions, bone powder, ear wax, or combinations thereof.

In another embodiment, the sample may be contained within the test subject, which may be an experimental animal (e.g. a mouse or rat) or a human, wherein the method is an in vivo based test. Alternatively, the sample may be an ex vivo sample or an in vitro sample. Therefore, the cells being tested may be in a tissue sample (for ex vivo based tests) or the cells may be grown in culture (an in vitro sample). Preferably, the biological sample is an ex vivo sample.

The method may comprise detecting the expression (or presence), in the sample, of the at least one senescent cell biomarker, wherein an increased level of expression of the at least one biomarker or a variant or fragment thereof, relative to the level of expression detected in the reference sample is an indication of a senescent cell present in the sample. Preferably, the reference sample (i.e. a control) does not comprise any senescent cells, and so does not express any of the biomarkers, or only very low or undetectable concentrations thereof.

Expression of at least one of the senescence biomarkers or a variant or fragment thereof may be detected in any compartment of the cell (e.g. in the nucleus, cytosol, the Endoplasmic Reticulum, the Golgi apparatus or the intracellular surface of the plasma membrane), or on the cell surface. Preferably, the senescence biomarker is expressed on the cell surface or physically associated with the intracellular or extracellular surface of the plasma membrane.

The inventors have developed a kit which is useful for detecting senescent cells.

According to a third aspect, there is provided a senescent cell detection kit for detecting senescent cells in a sample, the kit comprising means for detecting the presence, in a sample from a test subject, of at least one senescent cell biomarker selected from DEP-1, NTAL, EBP50, STX4, VAMP3, ARMCX-3, LANCL1, B2MG, PLD3 and VPS26A, or a variant or fragment thereof.

It will be appreciated that the inventors have determined that there are ten biomarkers which are associated with senescence, and the kits of the invention may comprise means for detecting one or more of the senescence biomarkers, or a variant or fragment thereof. The kit may therefore comprise means for detecting: DEP-1, NTAL, EBP50, STX4, VAMP3, ARMCX-3, LANCL1, B2MG, PLD3 and VPS26A, or a variant or fragment thereof, or combinations thereof.

Preferably, the kit comprises at least one control or reference sample. The kit may comprise a negative control and/or a positive control. A negative control may comprise any non-senescent cell that does not express any of the senescent biomarkers according to the invention, or only very low or undetectable concentrations thereof. A positive control may comprise any senescent cell that does express one or more of the senescent biomarkers according to the invention.

Senescent biomarkers according to the invention, which do not contain an extracellular domain may be detected using conventional techniques known in the art that are capable of detecting intracellular expression of a protein, such as Western Blots, immuno-precipitation or flow cytometry (with the aid of a permeabilisation agent, such as saponin).

The kit may comprise a means to compare the level of expression (or concentration) of the senescent biomarkers in the negative control sample to the level of expression of the equivalent biomarkers in a biological sample from an unknown subject, wherein an increased level of expression of one or more of the biomarkers relative to that detected in the negative control is an indication of senescence in the sample. Hence, by way of example, the concentration of the biomarker or a fragment or variant thereof in a sample with a senescent cell may be at least 1-, 2-, 5- or 10-fold high than in the negative control.

The inventors believe that the various senescence cell biomarkers described herein can be harnessed in a cell targeting strategy for specifically targeting and then killing senescent cells.

As such, according to a fourth aspect, there is provided a senescent cell biospecific drug conjugate for killing a senescent cell, the conjugate comprising a senescent cell targeting agent configured, in use, to specifically target and bind to at least one senescent cell biomarker selected from DEP-1, NTAL, EBP50, STX4, VAMP3, ARMCX-3, LANCL1, B2MG, PLD3 and VPS26A, or a variant or fragment thereof, and a cytotoxic agent, which kills the bound senescent cell.

The senescent cell targeting agent may be an antibody or an antigen binding fragment thereof, or an aptamer. Antibodies and fragments thereof represent preferred agents for use according to the invention. Antibodies according to the invention may be produced as polyclonal sera by injecting antigen into animals thereby producing polyclonal antibodies. Preferred polyclonal antibodies may be raised by inoculating an animal (e.g. a rabbit) with antigen using techniques known to the art. Preferably, however, the antibody is a monoclonal antibody. Antibodies according to the invention may also comprise plastic antibodies. The term “plastic antibody” can mean molecularly imprinted polymer nanoparticles (MIPs) with affinity for a target peptide or protein. When monomers are polymerised in the presence of the selected molecular target, collective weak interactions between the monomers and the target during polymerization result in the formation of populations of complementary binding sites in the resulting polymer. This molecular imprinting approach has been previously used to target biologically relevant molecules, including peptides and proteins. Binding affinity and selectivity of MIPs can be comparable to those of natural antibodies and they have previously been shown to be effective in vivo. Moreover, MIPs can be conjugated with the desired cytotoxic drugs and this approach has also been shown to be efficient in the delivery of such toxic payloads to cells.

Preferred antibodies and epitope binding fragments thereof may have immunospecificity for any of the senescence biomarkers according to the invention. Antibodies according to the invention may therefore be raised against any one or more of SEQ ID Nos. 1-19, or a fragment or variant thereof.

Preferably, SEQ ID Nos. 2, 4, 9, 10, 12, 13, 14, 16 or 18, or a fragment or variant thereof may be used as an antigen to create antibodies that specifically bind to senescent cells that display or express an extracellular biomarker according to the invention.

Functionally equivalent derivatives of the antibodies of the invention are also encompassed and may comprise at least 75% sequence identity, more preferably at least 90% sequence identity, and most preferably at least 95% sequence identity. It will be appreciated that most sequence variation may occur in the framework regions (FRs), whereas the sequence of the complementarity determining regions (CDRs) of the antibodies should be most conserved. For introduction into humans, the antibody may be humanised, by splicing V region sequences (e.g. from a monoclonal antibody generated in a non-human hybridoma) with a C region (and ideally FRs from the V region) sequences from human antibodies. The resulting ‘engineered’ antibodies are less immunogenic in humans than the non-human antibodies from which they were derived and so are better suited for clinical use.

Preferably, the FR region of the antibody is conjugated or fused with the cytotoxic agent, which may comprise a radioisotope, a toxin or a toxic peptide. The isotope may be any one selected from ¹³¹I or ⁹⁰Y. The toxin may be doxorubicin, calicheamicin, auristatin, maytansinoid, duocarmycin, or camptothecin analogues. The toxic peptide may be Pseudomonas exotoxin A, diphtheria toxin, ricin, gelonin, saporin or pokeweed an antiviral protein.

Antibodies according to the invention specifically kill senescent cells due to their ability to specifically bind to senescent cells via their CDR region(s). Therefore, the drug conjugate of the fourth aspect, and preferably antibodies according to this aspect, may be use to treat or delay the onset of age-related diseases.

In another embodiment, the targeting agent of the drug conjugate may be a small molecule. The small molecule may be capable of specifically binding to an epitope of a biomarker that is expressed or displayed on the surface of senescent cells. In another embodiment, the small molecule may comprise a means for gaining entry into senescent cells and specifically binding to an epitope of a biomarker that is expressed intracellularly. The small molecule may have a weight of less than 1000 Da.

It will be appreciated that drug conjugates according to this aspect of the invention may be used to specifically target and kill senescent cells that express or display senescent cell biomarkers, according to the invention, on the intracellular or extracellular surface of their plasma membrane.

In embodiments where the drug conjugate is intended to target a biomarker, which is expressed or displayed on the extracellular surface of senescent cells, the targeting agent of the drug conjugate may be an antibody comprising CDRs that specifically binds to an extracellular epitope of the biomarker.

In embodiments where the drug conjugate is intended to target a senescent cell biomarker, which is only expressed intracellularly, the targeting agent of the drug conjugate may be a small molecule that is capable of gaining entry into senescent cells and specifically binding to an epitope of the biomarker.

The senescent cell biomarkers according to the invention may be used to identify senescent cells, which can be targeted, for example, by the drug conjugate according to this aspect of the invention, for treatment of conditions associated with cell senescence, such as ageing and cancer.

Hence, in a fifth aspect, there is provided the senescent cell biospecific drug conjugate according to the fourth aspect, for use as a medicament.

In a sixth aspect, there is provided the senescent cell biospecific drug conjugate according to the fourth aspect, for use in treating, preventing or ameliorating an age-related disease.

In a seventh aspect, there is provided a method of treating, preventing or ameliorating an age-related disease, the method comprising administering, to a subject in need of such treatment, a therapeutically effective amount of the senescent cell biospecific drug conjugate according to the fourth aspect.

Age-related diseases may include but are not limited to impaired wound healing, dermal thinning, arterial wall stiffening, atherosclerosis, cardiovascular disease, cancer, arthritis, glaucoma, cataracts, osteoporosis, type 2 diabetes, hypertension, Alzheimer's disease and other types of dementia.

According to an eighth aspect, there is provided an age-related disease treatment pharmaceutical composition comprising the senescent cell biospecific drug conjugate according to the fourth aspect and a pharmaceutically acceptable vehicle.

According to a ninth aspect, there is provided a method of specifically killing senescent cells, the method comprising:

-   -   (i) determining the presence of a senescence cell in a subject;         and     -   (ii) administering, to a subject, a therapeutically effective         amount of the senescent cell biospecific drug conjugate         according to the fourth aspect.

The method of the ninth aspect of the invention may be used to specifically kill senescent cells in vivo, in vitro or ex vivo.

The methods, kits, conjugates and compositions according to the invention preferably comprise the use of at least one of the polypeptide sequences substantially as set out in any one of SEQ ID Nos. 1 to 19, or a fragment or variant thereof, as a biomarker of a senescent cell. Preferably, at least one of the polypeptide sequences substantially as set out in any one of SEQ ID Nos. 2, 4, 9, 10, 12, 13, 14, 16 or 18, or a fragment or variant thereof is used as an extracellular biomarker of senescent cells.

It will be appreciated that agents, conjugates, antibodies and compositions according to the invention may be used in a medicament which may be used in a monotherapy, for treating or delaying the onset of age-related diseases. Alternatively, such agents according to the invention may be used as an adjunct to, or in combination with, known therapies for treating or delaying the onset of age-related diseases.

The agents and antibodies according to the invention may be combined in compositions having a number of different forms depending, in particular, on the manner in which the composition is to be used. Thus, for example, the composition may be in the form of a powder, tablet, capsule, liquid, ointment, cream, gel, hydrogel, aerosol, spray, micellar solution, transdermal patch, liposome suspension or any other suitable form that may be administered to a person or animal in need of treatment. It will be appreciated that the vehicle of medicaments according to the invention should be one which is well-tolerated by the subject to whom it is given.

Medicaments comprising agents and antibodies according to the invention may be used in a number of ways. For instance, oral administration may be required, in which case the agents may be contained within a composition that may, for example, be ingested orally in the form of a tablet, capsule or liquid. Compositions comprising agents of the invention may be administered by inhalation (e.g. intranasally). Compositions may also be formulated for topical use. For instance, creams or ointments may be applied to the skin.

Agents, compositions and antibodies according to the invention may also be incorporated within a slow- or delayed-release device. Such devices may, for example, be inserted on or under the skin, and the medicament may be released over weeks or even months. The device may be located at least adjacent to the treatment site. Such devices may be particularly advantageous when long-term treatment with agents used according to the invention is required and which would normally require frequent administration (e.g. at least daily injection).

In a preferred embodiment, agents, compositions and antibodies according to the invention may be administered to a subject by injection into the blood stream or directly into a site requiring treatment. For example, the medicament may be injected at least adjacent to a senescent cell, or within a tumour. Injections may be intravenous (bolus or infusion) or subcutaneous (bolus or infusion), or intradermal (bolus or infusion).

It will be appreciated that the amount of the agent, composition and antibody that is required is determined by its biological activity and bioavailability, which in turn depends on the mode of administration, the physicochemical properties of the modulator and whether it is being used as a monotherapy or in a combined therapy. The frequency of administration will also be influenced by the half-life of the agent or antibody within the subject being treated. Optimal dosages to be administered may be determined by those skilled in the art, and will vary with the particular agent in use, the strength of the pharmaceutical composition, the mode of administration, and the advancement of the senescence-associated disease(s). Additional factors depending on the particular subject being treated will result in a need to adjust dosages, including subject age, weight, gender, diet, and time of administration.

Generally, a daily dose of between 0.01 μg/kg of body weight and 500 mg/kg of body weight of the agent according to the invention may be used for treating, ameliorating, or preventing senescence-associated disease, depending upon which agent is used. More preferably, the daily dose is between 0.01 mg/kg of body weight and 400 mg/kg of body weight, more preferably between 0.1 mg/kg and 200 mg/kg body weight, and most preferably between approximately 1 mg/kg and 100 mg/kg body weight.

The agent, composition or antibody may be administered before, during or after onset of the senescence-associated disease. Daily doses may be given as a single administration (e.g. a single daily injection). Alternatively, the agent may require administration twice or more times during a day. As an example, agents may be administered as two (or more depending upon the severity of the disease being treated) daily doses of between 25 mg and 7000 mg (i.e. assuming a body weight of 70 kg). A subject receiving treatment may take a first dose upon waking and then a second dose in the evening (if on a two dose regime) or at 3- or 4-hourly intervals thereafter. Alternatively, a slow release device may be used to provide optimal doses of agents according to the invention to a patient without the need to administer repeated doses.

Known procedures, such as those conventionally employed by the pharmaceutical industry (e.g. in vivo experimentation, clinical trials, etc.), may be used to form specific formulations comprising the agents according to the invention and precise therapeutic regimes (such as daily doses of the agents and the frequency of administration).

A “therapeutically effective amount” of agent is any amount which, when administered to a subject, is the amount of the agent, the composition or antibody that is needed to treat the senescence-associated disease, or produce the desired effect, such as inhibiting senescence cell formation.

For example, the therapeutically effective amount of agent used may be from about 0.01 mg to about 800 mg, and preferably from about 0.01 mg to about 500 mg. It is preferred that the amount of agent is an amount from about 0.1 mg to about 250 mg, and most preferably from about 0.1 mg to about 20 mg.

A “pharmaceutically acceptable vehicle” as referred to herein, is any known compound or combination of known compounds that are known to those skilled in the art to be useful in formulating pharmaceutical compositions.

In one embodiment, the pharmaceutically acceptable vehicle may be a solid, and the composition may be in the form of a powder or tablet. A solid pharmaceutically acceptable vehicle may include one or more substances which may also act as flavouring agents, lubricants, solubilisers, suspending agents, dyes, fillers, glidants, compression aids, inert binders, sweeteners, preservatives, dyes, coatings, or tablet-disintegrating agents. The vehicle may also be an encapsulating material. In powders, the vehicle is a finely divided solid that is in admixture with the finely divided active agents according to the invention. In tablets, the active agent (e.g. the peptide or antibody) may be mixed with a vehicle having the necessary compression properties in suitable proportions and compacted in the shape and size desired. The powders and tablets preferably contain up to 99% of the active agents. Suitable solid vehicles include, for example calcium phosphate, magnesium stearate, talc, sugars, lactose, dextrin, starch, gelatin, cellulose, polyvinylpyrrolidone, low melting waxes and ion exchange resins. In another embodiment, the pharmaceutical vehicle may be a gel and the composition may be in the form of a cream or the like.

However, the pharmaceutical vehicle may be a liquid, and the pharmaceutical composition is in the form of a solution. Liquid vehicles are used in preparing solutions, suspensions, emulsions, syrups, elixirs and pressurized compositions. The active agent according to the invention may be dissolved or suspended in a pharmaceutically acceptable liquid vehicle such as water, an organic solvent, a mixture of both or pharmaceutically acceptable oils or fats. The liquid vehicle can contain other suitable pharmaceutical additives such as solubilisers, emulsifiers, buffers, preservatives, sweeteners, flavouring agents, suspending agents, thickening agents, colours, viscosity regulators, stabilizers or osmo-regulators. Suitable examples of liquid vehicles for oral and parenteral administration include water (partially containing additives as above, e.g. cellulose derivatives, preferably sodium carboxymethyl cellulose solution), alcohols (including monohydric alcohols and polyhydric alcohols, e.g. glycols) and their derivatives, and oils (e.g. fractionated coconut oil and arachis oil). For parenteral administration, the vehicle can also be an oily ester such as ethyl oleate and isopropyl myristate. Sterile liquid vehicles are useful in sterile liquid form compositions for parenteral administration. The liquid vehicle for pressurized compositions can be a halogenated hydrocarbon or other pharmaceutically acceptable propellant.

Liquid pharmaceutical compositions, which are sterile solutions or suspensions, can be utilised by, for example, intramuscular, intrathecal, epidural, intraperitoneal, intravenous and particularly subcutaneous injection. The agent, composition or antibody may be prepared as a sterile solid composition that may be dissolved or suspended at the time of administration using sterile water, saline, or other appropriate sterile injectable medium.

The agents and compositions of the invention may be administered orally in the form of a sterile solution or suspension containing other solutes or suspending agents (for example, enough saline or glucose to make the solution isotonic), bile salts, acacia, gelatin, sorbitan monoleate, polysorbate 80 (oleate esters of sorbitol and its anhydrides copolymerized with ethylene oxide) and the like. The agent, antibody or composition according to the invention can also be administered orally either in liquid or solid composition form. Compositions suitable for oral administration include solid forms, such as pills, capsules, granules, tablets, and powders, and liquid forms, such as solutions, syrups, elixirs, and suspensions. Forms useful for parenteral administration include sterile solutions, emulsions, and suspensions.

It will be appreciated that the invention extends to any nucleic acid or peptide or variant, derivative or analogue thereof, which comprises substantially the amino acid or nucleic acid sequences of any of the sequences referred to herein, including variants or fragments thereof. The terms “substantially the amino acid/nucleotide/peptide sequence”, “variant” and “fragment”, can be a sequence that has at least 40% sequence identity with the amino acid/nucleotide/peptide sequences of any one of the sequences referred to herein, for example 40% identity with the polypeptide identified as SEQ ID Nos. 1-19, and so on.

Amino acid/polynucleotide/polypeptide sequences with a sequence identity which is greater than 50%, more preferably greater than 65%, 70%, 75%, and still more preferably greater than 80% sequence identity to any of the sequences referred to are also envisaged. Preferably, the amino acid/polynucleotide/polypeptide sequence has at least 85% identity with any of the sequences referred to, more preferably at least 90%, 92%, 95%, 97%, 98%, and most preferably at least 99% identity with any of the sequences referred to herein.

The skilled technician will appreciate how to calculate the percentage identity between two amino acid/polynucleotide/polypeptide sequences. In order to calculate the percentage identity between two amino acid/polynucleotide/polypeptide sequences, an alignment of the two sequences must first be prepared, followed by calculation of the sequence identity value. The percentage identity for two sequences may take different values depending on:—(i) the method used to align the sequences, for example, ClustalW, BLAST, FASTA, Smith-Waterman (implemented in different programs), or structural alignment from 3D comparison; and (ii) the parameters used by the alignment method, for example, local vs global alignment, the pair-score matrix used (e.g. BLOSUM62, PAM250, Gonnet etc.), and gap-penalty, e.g. functional form and constants.

Having made the alignment, there are many different ways of calculating percentage identity between the two sequences. For example, one may divide the number of identities by: (i) the length of shortest sequence; (ii) the length of alignment; (iii) the mean length of sequence; (iv) the number of non-gap positions; or (iv) the number of equivalenced positions excluding overhangs. Furthermore, it will be appreciated that percentage identity is also strongly length dependent. Therefore, the shorter a pair of sequences is, the higher the sequence identity one may expect to occur by chance.

Hence, it will be appreciated that the accurate alignment of protein or DNA sequences is a complex process. The popular multiple alignment program ClustalW (Thompson et al., 1994, Nucleic Acids Research, 22, 4673-4680; Thompson et al., 1997, Nucleic Acids Research, 24, 4876-4882) is a preferred way for generating multiple alignments of proteins or DNA in accordance with the invention. Suitable parameters for ClustalW may be as follows: For DNA alignments: Gap Open Penalty=15.0, Gap Extension Penalty=6.66, and Matrix=Identity. For protein alignments: Gap Open Penalty=10.0, Gap Extension Penalty=0.2, and Matrix=Gonnet. For DNA and Protein alignments: ENDGAP=−1, and GAPDIST=4. Those skilled in the art will be aware that it may be necessary to vary these and other parameters for optimal sequence alignment.

Preferably, calculation of percentage identities between two amino acid/polynucleotide/polypeptide sequences may then be calculated from such an alignment as (N/T)*100, where N is the number of positions at which the sequences share an identical residue, and T is the total number of positions compared including gaps but excluding overhangs. Hence, a most preferred method for calculating percentage identity between two sequences comprises (i) preparing a sequence alignment using the ClustalW program using a suitable set of parameters, for example, as set out above; and (ii) inserting the values of N and T into the following formula:—Sequence Identity=(N/T)*100.

Alternative methods for identifying similar sequences will be known to those skilled in the art. For example, a substantially similar nucleotide sequence will be encoded by a sequence which hybridizes to any sequences referred to herein or their complements under stringent conditions. By stringent conditions, we mean the nucleotide hybridises to filter-bound DNA or RNA in 3× sodium chloride/sodium citrate (SSC) at approximately 45° C. followed by at least one wash in 0.2×SSC/0.1% SDS at approximately 20-65° C. Alternatively, a substantially similar polypeptide may differ by at least 1, but less than 5, 10, 20, 50 or boo amino acids from the sequences shown in SEQ ID Nos.1-19.

Due to the degeneracy of the genetic code, it is clear that any nucleic acid sequence described herein could be varied or changed without substantially affecting the sequence of the protein encoded thereby, to provide a variant thereof. Suitable nucleotide variants are those having a sequence altered by the substitution of different codons that encode the same amino acid within the sequence, thus producing a silent change. Other suitable variants are those having homologous nucleotide sequences but comprising all, or portions of, sequence, which are altered by the substitution of different codons that encode an amino acid with a side chain of similar biophysical properties to the amino acid it substitutes, to produce a conservative change. For example small non-polar, hydrophobic amino acids include glycine, alanine, leucine, isoleucine, valine, proline, and methionine. Large non-polar, hydrophobic amino acids include phenylalanine, tryptophan and tyrosine. The polar neutral amino acids include serine, threonine, cysteine, asparagine and glutamine. The positively charged (basic) amino acids include lysine, arginine and histidine. The negatively charged (acidic) amino acids include aspartic acid and glutamic acid. It will therefore be appreciated which amino acids may be replaced with an amino acid having similar biophysical properties, and the skilled technician will know the nucleotide sequences encoding these amino acids.

All of the features described herein (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined with any of the above aspects in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying Figures, in which:—

FIG. 1 shows the analysis of the membrane fraction of EJp16 and EJp21 cells without and with induction of p16 and p21 respectively. A) Western blots of EJp16 and EJp21 in the presence or absence of tet (tetracycline), showing the induction of expression of exogenous p16 or p21, respectively. B) SA-β-Gal staining of EJp16 and EJp21 cells uninduced (Control) or 4 days after tet removal to induce expression of exogenous p16 or p21 (Senescent). Blue staining and morphological changes are indicative of senescence. C) Western blot analysis of lysates separated into cytosolic and membrane fractions of EJp21 and EJp16 cells uninduced (C) or 4 days after tet removal (S). Calnexin is used as a marker of membrane proteins and MAPK as a marker of the cytosolic fraction;

FIG. 2 shows proteomic screening of membrane proteins in senescent cells. A) Graphic representation of mass spec hits in EJp21 and EJp16 control and senescent cells. B) Number of membrane proteins differentially expressed in control and senescent EJp21 and EJp16, compared to those present in both conditions.

FIG. 3 is a Western blot validation of senescent-specific targets. A) and B) show protein expression of selected targets in the membrane fraction of lysates from EJp16 and EJp21 uninduced (C) or 4 days after tet removal (S). Calnexin and Na/K ATPase are used as loading controls;

FIG. 4 shows the expression of selected targets in membranes of senescent cells. Sucrose gradient fractionation of the membrane fraction of lysates from EJp164 days after tet removal. Calnexin and Na/K ATPase are used as markers of the cell membrane fractions. HDAC1 is used as marker of the nuclear fraction. MAPK is used as marker of the cytosolic fractions. SOD is used as marker of the mitochondrial fraction;

FIG. 5 shows the expression and localization of the novel senescence markers. Immunofluorescent images of selected targets in EJp16 and EJp21 uninduced (Control) or4 days after tet removal (Senescent), as well as early passage normal human diploid fibroblasts compared to those entering replicative senescence;

FIG. 6 relates to defining a new protocol for the detection of senescent cells. A) Representative plot analysis of fluorescence levels in control and senescent EJp16, HT1080p21-9 and human diploid fibroblasts (HDF) stained with fluorescently tagged antibodies against B2MG, DEP-1 and NOTCH3, as measured by flow cytometry. Senescent cells were analysed after 5 days of p16 or p21 expression. B) Average fold increases of mean fluorescence intensity (MFI) of the same cells. Experiments were performed in triplicate. Error bars show standard deviation; and

FIG. 7 is SA-β-Gal staining of control and senescent IMR90, HT1080p21-9 (after 4 days of p21 expression following exposure to IPTG) and normal human diploid fibroblasts (HDF).

EXAMPLES

The inventors have studied the expression profile of plasma membrane proteins in senescent cells in order to identify novel markers that could be easily recognized and propose potential effectors and modulators of the senescent pathway. Ten novel specific markers of senescence were validated (Examples 1 and 2), and two of these were selected in order to develop a fast and straightforward FACS-based approach to identify senescent cells (Example 3).

Materials and Methods

Cell Culture

The EJ human bladder cancer cell lines were maintained in DMEM supplemented with 10% fetal bovin serum (FBS) (Gibco), and pen-strep (50 unit/ml). EJ p21 and EJp53 cells were maintained with hygromycin (100 μg/ml) and genticin (750 μg/ml) plus (1 μg/ml) tetracycline. EJp16 cells were maintained with hygromycin (100 μg/ml) and puromycin (2 μg/ml) plus (1 μg/ml) tetracycline. In order to inhibit p21 and p16 expression, tetracycline (tet) was added to the medium every 3 days to final concentration (1 μg/ml). To induce p21, p16 and p53 expression, cells were washed three times and seeded directly in culture medium in the absence of tet (Fang et al., 1999). IMR90 (human fibroblasts wad derived from lungs of a 16-weeks female fetus) and 501T (human fibroblast which is driven from normal human skin) these fibroblasts were cultured until they reached the end of their replicative senescence. Restrictive dermopathy (RD) cells were kindly provided by Dr Sue Shackleton. To induce p21 expression in HT1080p21, 100 μM IPTG was added to the medium.

Plasma Membrane Protein Extraction

This method was performed according to the Abcam Plasma Membrane Protein extraction Kit (ab65400).

SDS-PAGE Separation

Senescent and growing EJp21 and EJp16 plasma membrane samples were separated by 10% SDSPAGE. After staining with the Coomassie blue, the gel was cut to obtain separate sample lanes. Each gel strip was then sliced into 50 slices, from the loading well down to the bottom of the gel. The proteins in the gel bands were digested with trypsin according to the protocol described previously (Shevchenko et al, 2006).

Extraction and Analysis of Proteins from Gel Lanes by Mass Spectrometry (Synapt G2S).

Gel lanes were cut sequentially into slices of approximately 1.5 mm and transferred to a 96 well low binding PCR plate. Each slice was washed/swollen with ammonium bicarbonate (80 ul, 50 mM) for 30 minutes, after this time the buffer was aspirated off using a Gilson. Each slice was destained with acetonitrile (80 ul) for 30 minutes, the solvent was removed. Steps 2) and 3) were repeated. After aspiration of the final acetonitrile, 15 ul of sequencing grade modified trypsin V511 (Promega), 20 ug/1.8 ml 25 mM ammonium bicarbonate, was added to each dehydrated gel piece. The plate was sealed and heated at 30° C. overnight. The sealing film was removed and extraction buffer added to each well (80 μl, 97% TFA (0.2%) 3% acetonitrile). The samples were extracted at room temperature for 1 hour. The extracted samples were transferred to low-binding eppendorf tubes and concentrated to dryness in a speedvac. The samples were redissolved in injection solvent (40 ul, 5% TFA) and analysed by mass spectrometry. Nanoscale LC was used to separate the complex peptide mixtures using a Waters nanoACQUITY UPLC. Chromatography was performed using a 50 minute reversed phase gradient (formic acid (0.1%)/acetonitrile) and a 75 μm×25 cm C18 column (Waters, BE130) operated at 300 nL/min. Mass spectrometry analysis was performed using a SYNAPT G25 (Waters Manchester UK) operated in a data-independent (MSE) manner. The selected analysis mode enabled precursor and fragment ions from the tryptic digest to be analysed simultaneously. The data acquired was processed and searched using ProteinLynx Global Server (Waters) and visualized and reanalyzed using Scaffold (Proteome Software, Oregon, USA).

Senescence-Associated-β-Galactosidase (SA-β-Gal) Staining

Cells were washed three times with PBS, and fixed with 4% formaldehyde for 5 min at room temperature. The detail of SA-β-gal staining was described previously (Dimri et al, 1995).

Immunoblot Analysis

Extracellular membrane samples were extracted and 1 μg/ml Protease Inhibitor Cocktail Set III (Calbiochem) added to the samples. Protein concentrations were then determined using Bradford protein assay (Fermentas). 20 μg of total cell protein per sample were subjected to 10% or 6% SDS-PAGE and transferred to Immobilon-P membrane (Millipore). An ECL detection system (Thermo Scientific) was used.

Immunofluorescence

Cells were split into 6-well plates containing sterile coverslips. After 24 hours, media was aspirated from the plates and cells were washed three times with 1×PBS. Cells were fixed using 1 ml of 4% paraformaldehyde for 30 min with gentle shaking. After fixing, cells were washed three times with 1×PBS and permeabilised with 1 ml 0.1% Triton X-100 for 10 minutes. Cells were then washed three times with 1×PBS and blocked with 1% BSA for 30 minutes. Coverslips were incubated with 100 μl 1:100 primary antibody overnight at 4° C. The following day, coverslips were washed three times with 1×PBS and incubated with 100 μl secondary anti-rabbit and anti-mouse antibody (Alexa Fluor 488 and 594, Invitrogen) for 45 minutes in the dark. After incubation, coverslips were washed three times with 1×PBS and stained with 4′,6-Diamidino-2-Phenylindole, Dihydrochloride (DAPI, Invitrogen) for 10 minutes. Slides were labelled and the coverslips were mounted and sealed with transparent nail varnish. Slides were analysed using a Nokia TE300 semi-automatic microscope.

FACS Analysis

Plates were washed with cold 1×PBS, and then the cells were collected by gently scraping them in 0.5 ml cold 1×PBS on ice. Trypsin should not be used because it leads to loss of extracellular proteins. The cells were then spun down at 200 g for 5 min at 4° C. The supernatant was discarded, the cells were re-suspend in 200 μl of blocking buffer (0.5% BSA+1×PBS) and then they were incubated on ice for 15 min. The cells were transferred into a 96 rounded bottom multi-well plate and spun down at 500 g for 5 min at 4° C. Once again, the supernatant was discarded. The pellet was re-suspend in an Antibody Mix and incubated at 4° C. in the dark for 30-45 min. The cells were then washed twice with Blocking Buffer (150 μl per well) followed by a spin at 500 g for 5 min at 4° C. The supernatant was discarded and the pellet was re-suspended in 300-500 μl of Blocking Buffer. Cellular fluorescence was detected using a cytometer.

Example 1—Proteomic Analysis of the Expression of Membrane Proteins in Senescent Cells

In order to characterize the profile of proteins selectively expressed in the plasma membrane of senescent cells, the inventors used a bladder cancer cell line, EJ, with a tet-regulatable p21 or p16 expression system (see FIG. 1A). These cells, named EJp21 and EJp16, respectively (Fang et al, 1999; Macip et al, 2002), irreversibly senesce after prolonged expression of the induced protein (see FIG. 1B). The membrane fraction was isolated form lysates of these proteins (see FIG. 1C) and a mass spectrometry screen performed to compare the senescent cells to their non-senescent counterparts. As shown in FIG. 2, 107 proteins were exclusively present in membranes of senescent EJp21 and 132 in EJp16. From these lists, ten proteins were selected for further validation: DEP-1, NTAL, EBP50, STX4, VAMP-3, ARMX-3, B2MG, LANCL1, VPS26A and PLD3. They were all chosen because they had not previously been shown to be associated with senescence and are all plasma membrane-associated proteins. None of the selected proteins had known functions that could immediately predict their mechanistic involvement in the senescent pathway. Of note, the screen also detected DCR-2, Notch-3 and ICAM1, all of which had been previously associated with senescence, which confirms the suitability of the screening protocols used.

Example 2—Validation of Potential Membrane Markers of Senescent Cells

The inventors next confirmed that the ten selected proteins (listed in Example 1) were indeed expressed preferentially in the membranes of senescent cells. To this end, the cell membrane fraction of lysates from EJp16 and EJp21 that had been induced to senesce were used. As shown in FIG. 3A, basal levels of DEP-1, NTAL, EBP50, STX4, VAMP3 and ARMCX-3 were low in uninduced EJp16 cells. After 5 days of p16 expression, when cells are known to be irreversibly senescent (Macip et al, 2002), expression of these proteins was highly increased, except for STX4 and VAMP-3, which only showed a minor induction. DEP-1 and NTAL were notably expressed in EJp21 in basal conditions, but were still up-regulated after the p21 induction for 5 days. NTAL, EBP50, STX4, VAMP-3 and ARMCX-3 all had low basal levels and a substantial increase in expression after EJp21 entered senescence. As shown in FIG. 3B, B2MG, LANCL1 and VPS26A underwent moderate increases in response to p16, but not p21. Also, PLD3 did not show any expression change in any model tested. Finally, DCR-2 was shown to be induced in both p16- and p21-dependent senescence, as expected. All of these results together confirmed that five of the potential markers (DEP-1, NTAL, EBP50, STX4, VAMP-3 and ARMCX-3) were specifically expressed in senescent cells, although at different levels, and three more (B2MG, LANCL1 and VPS26A) were up-regulated only in p16-induced senescence.

The inventors further confirmed these results using cell fractionation of EJp16 cell lysates by sucrose gradient. FIG. 4 shows that DEP-1, NTAL, EBP50, STX4, ARMCX-3 and B2MG co-localize in the same fraction as cell membrane markers Na/K ATPase and Calnexin. This underscores the hypothesis that these proteins are present in membrane of senescent cells. Immunofluorescent microscopy was also used to study the expression and localization of these proteins (see FIG. 5). DEP-1, NTAL, EBP50 and STX4 showed induction in senescent EJp16, as compared to the positive control (DCR-2). VAMP-3 and ARMCX-3 also showed up-regulation, but at lower levels. In EJp21, all markers were significantly increased. The expression of these proteins in IMR90 human fibroblasts was also measured, comparing early passage cells to those induced to senesce after serial passaging (see FIG. 7). All the proteins tested showed low basal levels in growing fibroblasts and increased expression in senescent ones (see FIG. 5), confirming that they could be used as markers of replicative senescence in normal cells.

Example 3—Characterization of Senescence Markers by FACS Analysis

With the information from the validation experiments (i.e. Example 2), the inventors chose two of the novel membrane proteins (DEP-1 and B2MG) to define a simple and specific protocol, using flow cytometry, that would allow for the rapid detection of senescent cells in culture. DEP-1 and B2MG were initially chosen because they had large extracellular epitopes recognized by commercially available fluorescent-tagged antibodies. NOTCH3 was used as a positive control. All three antibodies were mixed and incubated with non-permeabilized cells (see Materials and Methods for protocol details). The total time needed to measure the presence of senescent cells in cell cultures was under 2 hours. As shown in FIG. 6, there was a consistent 2- to 3-fold increase in all of the markers in EJp16 after the induction of senescence. This result was confirmed using another model of p21-induced senescence HT1080p21-9 (Chang et al, 2000; Masgras et al, 2012) (see FIG. 7), which showed approximately a 3-fold increase in cell surface expression in each of the three markers. Moreover, normal human diploid fibroblasts that entered replicative senescence also showed up-regulation of the markers, although at lower levels (FIG. 6), which is consistent with a lower percentage of SA-β-Gal positive cells (see FIG. 7). These results confirm that the validated membrane markers of senescence from the inventors proteomic screen can be successfully used to determine the presence of senescent cells in culture and could provide a faster and more selective detection tool than those currently available.

DISCUSSION

Senescence is a well-defined cellular mechanism with a critical role in processes as diverse as cancer and ageing. Despite having been studied for decades, the mechanisms involved in senescence are not fully understood. One of the features of senescent cells that had not been previously characterized was the profile of expression of proteins on their surface. Such proteins have the potential to be especially relevant for three reasons. Firstly, these proteins could contribute to explaining how these cells interact with the microenvironment and also aid our understanding of the mechanisms of senescent cell clearance. This is important in the context of the tumour suppressor functions of senescence as well as its involvement in the symptoms associated with ageing (Baker et al, 2011). Secondly, specific cell membrane proteins with extracellular epitopes would be useful for rapidly detecting senescent cells in a laboratory environment. Given that the current protocols for these analyses are far from ideal, identifying extracellular epitopes of the senescent proteome could greatly improve this field of study. Finally, uncovering novel up-regulated proteins could enhance our understanding of the processes that determine the establishment and maintenance of the senescent phenotype.

Using a proteomics approach, the inventors identified and validated ten proteins expressed at higher levels in plasma membrane fractions of senescent cells than in controls. Six of the proteins have at least one extracellular domain or are associated with the plasma membrane. From their known functions, it is not immediately clear what role they could play in senescence. DEP-1 participates in cell adhesion, which could determine how senescent cells interact with their microenvironment. STX4 and VAMP-3 contribute to vesicle traffic in cells, perhaps impinging on some aspects of the SASP. NTAL, EBP50 and LANCL1 belong to different signalling pathways that could be linked to senescence. B2MG and VPS26A have roles in the immune system, and this could be related to the clearance of senescent cells from tissues. ARMCX-3 has a potential tumour suppressor effect that could perhaps be explained by its role in inducing senescence. Finally, PLD3's phospholipase activity may be involved in senescence through unknown mechanisms. Further experiments to determine whether any of these proteins actively contribute to the senescent phenotype (or if their upregulation is just an epiphenomenon) are currently being performed.

All 10 targets were studied in different models, mainly the inducible EJ cell lines that undergo senescence through activation of one of the main pathways involved in the process, p16 or p21. All of them were up-regulated in at least one of the models, with most clearly induced in both. Moreover, the results were also validated in normal human fibroblasts, thus confirming the relevance of the data in both replicative and stress-induced models of senescence.

The inventors have proven that these proteins, specifically the six that showed better induction (DEP-1, NTAL, EBP50, STX4, VAMP-3, ARMCX-3 and B2MG), have the potential to be used as surrogate markers of senescence, together with those previously described (p21, p16, p15, DCR2, NOTCH-3, etc.). As a proof of principle, they selected two of the six proteins, DEP-1 and B2MG, to develop a staining protocol that could help determine the amount of senescent cells present in a sample. The goal was to achieve higher specificity and shorter experimental times than the current gold standard, the SA-β-Gal assay. The inventors believe that their results show that such a detection method, based on specific antibodies against extracellular epitopes, is feasible and successful. Results can be obtained under 2 hours, compared with the overnight incubation times needed for the classic SA-β-galactosidase staining. Further optimization will be required to determine the best targets and conditions. Increasing the simultaneous number of markers detected could augment the specificity of the protocol, if needed. Also, markers more specific to either the p16 or p21 pathways could help determine which of the two pathways is preferentially activated in response to each senescence-inducing stimulus.

This proteomic screen provides new information about the mechanisms involved in senescence and can be used experimentally to rapidly detect senescent cells. Moreover, the inventors hope that further studies, in the future, will determine the exact role of these novel markers in the senescent pathways, thus contributing to our understanding of this intricate cellular process. Such information could be important to define new therapeutic interventions that could increase the positive impact of senescence on human health and/or diminish its negative effects.

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1. A method of detecting a senescent cell in a sample, the method comprising detecting the expression, in the sample, of at least one senescent cell biomarker selected from DEP-1, NTAL, EBP50, STX4, VAMP3, ARMCX-3, LANCL1, B2MG, PLD3 and VPS26A, or a variant or fragment thereof, wherein an increased level of expression of the at least one biomarker or a variant or fragment thereof relative to the level of expression detected in a reference sample is an indication of a senescent cell present in the sample.
 2. A method according to claim 1, wherein the at least one senescent cell biomarker comprises an amino acid sequence substantially as set out in any one of SEQ ID Nos. 1 to 19, or a variant or fragment thereof.
 3. A method according to claim 1, wherein the at least one senescent cell biomarker is DEP-1, NTAL, EBP50, STX4, VAMP-3, PLD3 or ARMCX-3, or a variant or fragment thereof.
 4. (canceled)
 5. A method according to claim 1, wherein one or more of DEP-1, NTAL, B2MG, ARMCX-3, PLD3 and LANCL1, or a variant or fragment thereof, is used as an extracellular biomarker.
 6. (canceled)
 7. (canceled)
 8. The method according to claim 1, wherein the method comprises detecting two, three, four or more biomarkers, or variants or fragments thereof, in the sample.
 9. The method according to claim 1, wherein the sample is a bodily sample taken from a test subject, and wherein the test subject is an experimental animal or a human.
 10. (canceled)
 11. (canceled)
 12. The method according to claim 1, wherein sample is an ex vivo sample or an in vitro sample.
 13. A senescent cell detection kit for detecting senescent cells in a sample, the kit comprising means for detecting the presence, in a sample from a test subject, of at least one senescent cell biomarker selected from DEP-1, NTAL, EBP50, STX4, VAMP3, ARMCX-3, LANCL1, B2MG, PLD3 and VPS26A, or a variant or fragment thereof.
 14. A kit according to claim 13, wherein the kit comprises means for detecting more than one of the senescence biomarkers, or a variant or fragment thereof, and at least one control or reference sample.
 15. (canceled)
 16. A kit according to claim 13, wherein the kit comprises a negative control and/or a positive control, wherein the negative control is any non-senescent cell that does not express any of the senescent biomarkers according to the invention, or only very low or undetectable concentrations thereof, and wherein a positive control comprises any senescent cell that does express one or more of the senescent biomarkers.
 17. (canceled)
 18. (canceled)
 19. A senescent cell biospecific drug conjugate for killing a senescent cell, the conjugate comprising a senescent cell targeting agent configured, in use, to specifically target and bind to at least one senescent cell biomarker selected from DEP-1, NTAL, EBP₅₀, STX₄, VAMP₃, ARMCX-3, LANCL₁, B₂MG PLD₃ and VPS₂₆A or a variant or fragment thereof, and a cytotoxic agent, which kills the bound senescent cell.
 20. A drug conjugate according to claim 19, wherein the targeting agent is an antibody or an antigen binding fragment thereof, an aptamer, plastic antibody, or a small molecule.
 21. (canceled)
 22. A drug conjugate according to claim 19, wherein the cytotoxic agent is a radioisotope, and wherein the radioisotope is either ¹³¹I or ⁹⁰Y, a toxin or a toxic peptide.
 23. A drug conjugate according to claim 22, wherein the cytotoxic agent is a toxin, and wherein the toxin is doxorubicin, calicheamicin, auristatin, maytansinoid, duocarmycin, or camptothecin analogues.
 24. A drug conjugate according to claim 22, wherein the cytotoxic agent is a toxic peptide, and wherein the toxic peptide is Pseudomonas exotoxin A, diphtheria toxin, ricin, gelonin, saporin or pokeweed antiviral protein.
 25. (canceled)
 26. (canceled)
 27. A method of treating, preventing or ameliorating an age-related disease, the method comprising administering, to a subject in need of such treatment, a therapeutically effective amount of the senescent cell biospecific drug conjugate according to claim
 22. 28. A method according to claim 27, wherein the age-related disease is atherosclerosis, cardiovascular disease, cancer, arthritis, glaucoma, cataracts, osteoporosis, type 2 diabetes, hypertension, Alzheimer's disease or other type of dementia.
 29. (canceled) 